This invention relates to the acquisition, at different levels in a borehole, of seismic signals generated by a source placed in the vicinity of the surface opening of the borehole.
When the seismic waves produced by a source are recorded by means of a detector at different levels in a borehole, a series of seismic signals is obtained which are gathered, after suitable processing, to form a vertical seismic profile. The analysis of such a profile provides precious information on the structure of the geological formations traversed by the borehole, and notably on the position and the dip of the reflecting layers, including those which are located at a depth from the surface greater than the borehole bottom.
For a complete analysis, the recordings must be carried out at a very large number of different levels, for example 200 or more. This takes a very long time, especially as several measurements are carried out at each level in order to improve the quality of the signals.